Boiler inlet plug insert with heat dams

ABSTRACT

A device for eliminating the thermal instability of a liquid in forced feed convection boilers in which subcooled liquids which are introduced at the inlet have a tendency to superheat and flash into vapor to produce liquid slugs which are discharged at the outlet with the dry or superheated vapor. The invention is a rod-or sleeve-type insert located in the heating coils which produces liquid flow damping and controls the liquid superheating, and incorporates a nucleation device or devices disposed axially along the direction of liquid flow and introduces heat dams at the nucleation sites created by vapor from the heated site to prevent the liquid being heated from entering the nucleation site and flashing into vapor.

United States Patent [72] Inventor Alfred Koestel Nogales, Ariz. [2]]Appl. No. 8,797 [22] Filed Feb. 5, 1970 [45] Patented May 25, 1971 [73]A ign The United States of America as represente by the Secretary of theAir Force [54] BOILER INLET PLUG INSERT WITH HEAT DAMS 5 Claims, 8Drawing Figs.

[52] US. Cl 122/40611, 165/135, 165/177 [51] Int. Cl F221) 37/34 [50]Field of Search 122/406, 406 (R); 165/134, 135, 174, 177

[56] References Cited UNITED STATES PATENTS 2,797,554 7/1957 Donovan165/174X 3,029,796 4/1962 Simmonsetal 3,336,974 8/1967 BemsteinetalPrimary Examiner-Kenneth W. Sprague Attorneys-Harry A. Herbert, Jr. andCharles H. Wagner ABSTRACT: A device for eliminating the thermalinstability of a liquid in forced feed convection boilers in whichsubcooled liquids which are introduced at the inlet have a tendency tosuperheat and flash into vapor to produce liquid slugs which aredischarged at the outlet with the dry or superheated vapor. Theinvention is a rod-or sleeve-type insert located in the heating coilswhich produces liquid flow damping and controls the liquid superheating,and incorporates a nucleation device or devices disposed axially alongthe direction of liquid flow and introduces heat dams at the nucleationsites created by vapor from the heated site to prevent the liquid beingheated from entering the nucleation site and flashing into vapor.

Tl/EBINE I Patented May 25, 1971 3,580,227

3 Sheets-Sheet 3 BOILER INLET PLUG INSERT WITH HEAT DAMS BACKGROUND OFTHE INVENTION Under certain conditions, liquid forced through heatingcoils of forced convection liquid boilers with subcooled liquid injectedat the inlet and with dry or superheated vapors discharged at the outletdevelops severe liquidboiling heat transfer problems at the localizedregion or regions in the heating zone where the boiling of the liquidbegins. One of these problems is a thermal instability in the liquidcaused by the tendency of the heated liquid to superheat and thenexplosively flash into vapor. This results in liquid slugs separated bylarge voids of vapor. This pressure pulsing produced by the flashing andthe low heat transfer rate due to the dry-heating surfaces in the vaporvoids are objectional features.

Superimposed on this phenomenon there can occur a hydrodynamic type ofinstability due to insufficient damping of the inlet flow.

SUMMARY OF THE INVENTION The invention is a device to eliminate orcontrol these problems and comprises a rodor sleeve-type insert, orinserts, located in the heating coils for the liquid which produces flowdamping and controls the liquid superheating, increasing the flowresistance in the heating coils by reducing the flow crosssectional areaand increasing the flow velocity. Also the invention provides stabilizednucleation sites located in the heating coils for controlling thethermal instabilities of the liquid resulting from superheating.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic sectional viewof a liquid heating unit incorporating the invention.

FIG. 2 is an enlarged sectional view taken through a portion of one ofthe liquid heating coils, more clearly illustrating the stabilizednucleation site and heat dam unit.

FIG. 3 is an enlarged transverse sectional view taken about on line 3-3of FIG. 2.

FIG. 4 is a transverse sectional view taken about on line 4-4 of FIG. 2.

FIG. 5 is a similar sectional view through the heat dam unit or inserttaken about on line 5-5 of FIG. 2.

FIG. 6 is an enlarged fragmentary longitudinal sectional view takenthrough the nucleation site opening of the heat dam" tubular unit.

FIG. 7 is a fragmentary, somewhat schematic sectional view showing a rodtype of heat dam with nucleation sites and fitted in the heating coil.

FIG. 8 is a fragmentary sectional view taken about on the planeindicated by line 8-8 of FIG. 7, portions being broken away.

Referring to the drawings, and more particularly to FIG. I whichschematically shows a liquid metal vapor power output unit and boilerincorporating the invention, in which the reference numeral 1 denotesthe unit generally, comprising the heating units 2 and 3 and theliquid-heating vaporizing coils 4 and 5 having a subcooled liquid inlet6 and the vaporized liquid outlet at 7.

Subcooled liquid is supplied to the inlet 6 at a predetermined unifonnrate by a suitable pump 8 while the vaporized liquid is discharged atthe vapor outlet 7 into and through a suitable engine or turbine 9having a power takeofF' shaft 10. After passing through the engine orturbine 9, the vaporized liquid under some predetermined pressure ispreferably passed through a suitable condenser or cooler unit 11 of anyconventional construction and returned through the conduit 12 to thepump unit 8 where it is again circulated through the heating coils 4 and5, and vaporized by the liquid heaters 2 and 3 and discharged throughthe vapor outlet end 7 from the heating coils 4 into the turbine unit 9to continue to drive the power takeoff shaft 10.

It has been found that under certain conditions, for instance oncethrough forced feed, convection boilers, with subcooled liquid injectedat the inlet and with dry or superheated vapor discharged at the outlet,develop severe boiling heat transfer problems, affecting reliableoperations, and these problems are localized in a region in which theboiling begins.

One of these problems is a thermal instability caused by the tendency ofthe liquid to superheat, and then explosively flash into vapor. Thisresults in liquid slugs separated by large voids of vapor discharged atthe vapor outlet 7. The pressure pulsing produced by this flashing, andthe low heat transfer rates due to the dry-heating surfaces in the vaporvoids are the objectional features. Superimposed upon this phenomenonthere can occur a hydrodynamic type of instability due to insufficientdamping of the inlet flow. This is also an objectional phenomenon.

The invention described herein is a device and construction foreliminating or controlling these problems, and comprises rodorsleeve-type inserts in the heating coils 4 and/or 5, which produce flowdamping and control the liquid superheating. The invention increasesflow resistance by damping, by reducing the flow cross-sectional areaand increases the flow velocity. It also provides stabilized nucleationsites in the liquid heating coils 4 and 5 for controlling the thermalinstabilities resulting from liquid superheating. The nucleation devicescomprise nucleation sites which are stabilized by means ofheat dams."

Referring more particularly to the schematic sectional view in F. 2, oneof these heat darn units is indicated generally at 13 and, as shown inthis FIG. of the drawing, is in the form of a tubular insert or sleevehaving an inner bore 13' of predetermined diameter for restricting ordamping the rate of flow of the liquid being heated between liquid inletend of the tubular coil 4 or 5 and the heated vapor outlet end 7.

The tubular nucleation and heat dam members, or inserts 13, are suitablyinserted and secured in the heating coils 4 or 5 where the heated liquidis superheated to flash conditions by the heating units 2 or 3 and tendsto explosively flash into vapor, and constitutes nucleation sites 14.The inserts 13 are formed with cylindrical end support portions 15 ofsuitable length which are in contact with the inner walls 40 of theheating coils 4 or 5 to centralize the tubular heat dam units 13 in thecoils and provide heat-conducting means to the inner reduced diameter,tubular portions 16 extending between the spaced heads 15-15 and providea spaced relation between the inner surface of the liquid heating coils4 or 5 and the outer surface of the tubular portion 16, to form a heatdam or vapor chamber 17 at the nucleation sites 14. This space or cavity17 is so designed that the liquid vapor interface is bothhydrodynamically and thermally stable, and thus provides a heat dam" atthe nucleation sites 14 in which once the liquid in the coils 4 or 5enters through the nucleation site opening 18, and is first flashed intovapor by the first pressure pulse to fill the chamber 17, this vaporthen provides thermal resistance to create a dam" to the free flow ofheat. The result is a hot zone" which prevents reentry of the liquidinto the nucleation site. The hot zone 17 thereby acts as a dam andprevents the repetition of the flashing of the liquid into vapor, whichinitiated the heat dam" in the first place. The incorporation of thisprinciple, with a plug-type flow restriction 13, is an important objectof the invention.

Nucleation sites in forced convection boilers have been investigated invarious laboratories. Roughened surfaces and drilled holes have beenused as nucleation sites. In many cases, the sites were not effectivebecause the flashed" vapor was washed out" or collapsed, and there wasstill a need for liquid stabilization in the sites. The employment ofthe heat dam principle of the subject invention has satisfied thisrequirement, and in addition the inlet flow is stabilized by the plug orsleeve insert 13, incorporating the heat dam" therein.

The device becomes effective against various instabilities which havebeen found to plague forced convection boilers. The size of thenucleation site opening and the length of the chamber 17 between theheat dams 13 must be such that the vapor interface maintained in thesite or cavity 17 should be satisfied, and the vapor should not bewashed out" or collapsed due to flow or thermal disturbances. When theboiler is started, a single pulse" due to superheat is produced, in theliquid in the dam chamber 17, and this forms the vapor within the site.if this vapor is ejected or collapsed (condensed), the pulses willbecome repetitious thereby creating the objectionable thermalinstability. Some superheat in the vapor should, of course, bemaintained to prevent the collapse of the vapor in the cavity ll7 as aresult of the pressure pulse.

Referring to the sectional views shown in FIGS. 7 and 8, the

reference numeral denotes a portion of the liquid heating,

and vaporizing coil, for instance, at the location where the boiling ofthe liquid begins to take place. The portion of the coil is shownstraight for a better illustration thereof. The heat input means isomitted but comprises a suitable surrounding superheating unit topromote boiling and comprises heat transfer means for heating the liquidand superheating the vapor produced by the vaporization of the liquid,for instance, similar to heaters 2 and 3 in FIG. 1.

The reference numeral 2] denotes a rod-type insert which produces flowdamping and controls the liquid superheating and increases flowresistance (damping) by reducing the flow crosssectional area andincreasing the flow velocity, and also provides stabilized nucleationsites for controlling thermal instabilities resulting from liquidsuperheating.

In the drawing the reference numeral 23 denotes the central stem portionof the rod insert having the spaced heat transfer shoulders 24 and 25with the liquid flow passage 26 located therebetween.

Nucleation sites are indicated at 27, which extend across the heattransfer shoulders 24 and 25 from the liquid conducting flow passage 26into the vapor chamber 28, which provides a heat dam cavity between theenlarged portion 29 of the stem or rod 21 and the walls of the heattransfer shoulders 24 and 25 at the opposite sides of the vaporcollection cavity 28.

The heat dam cavity 28 provides a liquid vapor interface that is bothhydrodynamically and thermally stable. Some superheat in the vapor ismaintained in the cavity 28 to prevent collapse of the vapor in thecavity as the result of the initial pressure pulse. The vapor itselfcauses a resistance to the heat flow, thus raising the temperature ofthe surface and vapor. The amount of superheat for a stable heat dam"can be estimated by means of the well known Clausius-Clapyron relation.

The effect of the heat dam is created by vapor formed by the firstpressure pulse and flashing of the st perheated liquid in the cavity 28into vapor. After this the vapor thermal resistance in the cavity 28creates a dam to the flow of heat. The result is a hot zone whichprevents the reentry of the liquid from the flow passage 26 into thenucleation sites 27. The hot zone thereby prevents the repetition of theflashing of the liquid into vapor which initiated the heat dam" in thefirst place. The incorporation of this principle, as explained, with theplug" type restriction, as previously mentioned, is the basic invention.

For purposes of exemplification of the invention, a particularembodiment of the invention has been shown and described to the bestunderstanding thereof. However, it will be apparent that changes andmodifications in the arrangement and construction of the parts thereofmay be resorted to without departing from the true spirit and scope ofthe invention as defined in the accompanying claims.

lclaim:

1. In a liquid superheating and vaporizing device, liquid superheatingand vaporizing coil means having a subcooled inlet and a superheatedliquid vapor outlet and heating coil means between said inlet and saidoutlet for heating and vaporizing liquid in said coil means intennediatesaid inlet and said outlet, said liquid heating and vaporizing coilmeans having a nucleation site therein where boiling of said liquidbegins and tends to explosively flash into vapor, a heat dam" insertunit in said coil means enclosing said nucleation site having arestricted liquid flow passage between the ends thereof, in-

eluding spaced heat-conducting means in contact with the inner surfaceof the heating coil means having a closed heat dam" cavity thereinlocated between said spaced heat conducting means and the inner surfaceof said liquid heating and vaporizing coil means and the outer surfaceof said insert, and in surrounding relation to said nucleation site,said insert having a restricted liquid receiving opening therethroughbetween said liquid flow passage and the interior of said heat daminsert cavity for trapping and confining vapor of boilingliquid in saidcavity at said nucleation site to reduce heat transfer in said cavitybetween said heating coil means and the liquid at said site andincreasing the rate of liquid flow past said site, to prevent the liquidat said site, heated by said coil means, from explosively flashing intovapor during the flow of the liquid past said nucleation site.

2. in a liquid-superheating and -vaporizing device as set forth in claim1 in which said heat darn insert comprises an elongated cylindricalmember having longitudinally spaced heat transfer portions adapted to bedisposed in said heating coil means in heat transfer contact relationwith the inner surface of said heating coil means at opposite sides ofthe nucleation site in the heating coil means, said heat dam unit havinga smaller diameter elongated central portion between said end portionsto form a heat dam cavity between said end portions and the innersurface of said heating coil means and said smaller diameter elongatedcentral portion, said heat dam insert having an elongated restrictedcentral liquid flow passage therethrough between said spaced heattransfer portions to conduct liquid in said heating coil means throughsaid insert at an accelerated flow rate, said heat dam insert having arestricted vapor outlet therein communicating between the interior ofsaid heat dam" cavity and said restricted liquid flow passage, forinitially trapping and discharging heated liquid vapor from said heatdam cavity into said central liquid flow passage and resisting entranceof liquid from said flow passage into said heat dam cavity, for loweringheat transfer at said nucleation site between said heating coil meansand liquid in said central liquid flow passage passing said nucleationsite.

3. in a liquid-superheating and -vaporizing device as set forth in claiml in which said heat dam insert comprises a tubular member having acentral restricted liquid flow passage therethrough, longitudinallyspaced cylindrical heat transfer wall members disposed in heat transferrelation with the inner surface of said heating coil means and connectedto each other by a smaller diameter tubular connecting member havingsaid restricted liquid flow passage therethrough, to form an elongatedheat dam cavity between said cylindrical heat transfer wall members andthe inner surface of said heating coil means and the outer surface ofsaid smaller diameter tubular connecting member, said tubular memberhaving a restricted vapor vent passage therein extending from said heatdam cavity into said restricted liquid flow passage, for trapping liquidvapor from boiling liquid in said heat dam cavity, to prevent liquid insaid liquid flow passage from entering through said vapor vent passageinto said heat dam cavity, said heat dam insert adapted to be disposedin said heating coil means to surround a nucleation site in the heatingcoil means, and reduce transfer of heat at said nucleation site betweenthe heating coil means and the liquid in said liquid flow passage, toprevent said liquid from reaching said nucleation site and explosivelyflashing into vapor between said nucleation site and the superheatedvapor outlet in said heating coil means.

4. in a liquid-superheating and -vaporizing device as set forth in claim1, in which said heat dam insert comprises an elongated rod-type insertadapted to be inserted in said heating coil means having a restrictedliquid flow passage extending between the ends thereof, spaced elongatedheat transfer wall members projecting outwardly at opposite sides ofsaid liquid flow passage, between the opposite ends of said insert andadapted to be disposed in heat transfer relation with the inner surfaceof said heating coil means, said insert having an enlarged central stemmember between said heat transfer wall members having a smaller diameterthan the transverse inner diameter of said heating coil means, to form aheat dam cavity in said insert between said central stem member, theinner surface of said heating coil means, and said outwardly projectingheat transfer wall members, adapted to enclose a nucleation site in saidheating coil means, a restricted vapor vent passage in said insertextending across said heat transfer wall members in restrictedcommunication between the interior of said heat dam cavity and saidliquid flow passage, whereby said heat dam cavity in said insertsurrounding said nucleation site is adapted to trap liquid vapor fromboiling liquid in said heating coil means at said nucleation site andrestrict the inflow of liquid in said flow passage from entering saidheat dam cavity and explosively flashing into vapor at said nucleationsite.

5. A liquid-superheating and -vaporizing device as set forth in claim 4in which said spaced elongated heat transfer wall members extendhelically from end to end of said insert in uniform spaced relationaround said enlarged central stern portion with said restricted liquidflow passage extending helically around said central stem member betweensaid helically extending spaced elongated heat transfer wall members, toprovide a heat dam cavity helically surrounding said central stemmember, between said helical wall members and said enlarged central stemmember and the inner surface of said heating coil means, extendingacross said nucleation site in the heating coil means where liquid insaid heating coil means begins to boil and tends to explosively flashinto vapor, and a restricted venting passage in said helical dam cavityopening into said helical liquid flow passage between said spacedhelical wall members, for restricting the inflow of liquid from saidhelical liquid flow passage into said helical heat dam cavity by thevaporized liquid in said heat darn cavity.

1. In a liquid superheating and vaporizing device, liquid superheatingand vaporizing coil means having a subcooled inlet and a superheatedliquid vapor outlet and heating coil means between said inlet and saidoutlet for heating and vaporizing liquid in said coil means intermediatesaid inlet and said outlet, said liquid heating and vaporizing coilmeans having a nucleation site therein where boiling of said liquidbegins and tends to explosively flash into vapor, a ''''heat dam''''insert unit in said coil means enclosing said nucleation site having arestricted liquid flow passage between the ends thereof, includingspaced heat-conducting means in contact with the inner surface of theheating coil means having a closed ''''heat dam'''' cavity thereinlocated between said spaced heat conducting means and the inner surfaceof said liquid heating and vaporizing coil means and the outer surfaceof said insert, and in surrounding relation to said nucleation site,said insert having a restricted liquid receiving opening therethroughbetween said liquid flow passage and the interior of said heat daminsert cavity for trapping and confining vapor of boiling liquid in saidcavity at said nucleation site to reduce heat transfer in said cavitybetween said heating coil means and the liquid at said site andincreasing the rate of liquid flow past said site, to prevent the liquidat said site, heated by said coil means, from explosively flashing intovapor during the flow of the liquid past said nucleation site.
 2. In aliquid-superheating and -vaporizing device as set forth in claim 1 inwhich said heat dam insert comprises an elongated cylindrical memberhaving longitudinally spaced heat transfer portions adapted to bedisposed in said heating coil means in heat transfer contact relationwith the inner surface of said heating coil means at opposite sides ofthe nucleation site in the heating coil means, said heat dam unit havinga smaller diameter elongated central portion between said end portionsto form a ''''heat dam'''' cavity between said end portions and theinner surface of said heating coil means and said smaller diameterelongated central portion, said heat dam insert having an elongatedrestricted central liquid flow passage therethrough between said spacedheat transfer portions to conduct liquid in said heating coil meansthrough said insert at an accelerated flow rate, said heat dam inserthaving a restricted vapor outlet therein communicating between theinterior of said ''''heat dam'''' cavity and said restricted liquid flowpassage, for initially trapping and discharging heated liquid vapor fromsaid heat dam cavity into said central liquid flow passage and resistingentrance of liquid from said flow passage into said heat dam cavity, forlowering Heat transfer at said nucleation site between said heating coilmeans and liquid in said central liquid flow passage passing saidnucleation site.
 3. In a liquid-superheating and -vaporizing device asset forth in claim 1 in which said ''''heat dam'''' insert comprises atubular member having a central restricted liquid flow passagetherethrough, longitudinally spaced cylindrical heat transfer wallmembers disposed in heat transfer relation with the inner surface ofsaid heating coil means and connected to each other by a smallerdiameter tubular connecting member having said restricted liquid flowpassage therethrough, to form an elongated heat dam cavity between saidcylindrical heat transfer wall members and the inner surface of saidheating coil means and the outer surface of said smaller diametertubular connecting member, said tubular member having a restricted vaporvent passage therein extending from said heat dam cavity into saidrestricted liquid flow passage, for trapping liquid vapor from boilingliquid in said heat dam cavity, to prevent liquid in said liquid flowpassage from entering through said vapor vent passage into said heat damcavity, said heat dam insert adapted to be disposed in said heating coilmeans to surround a nucleation site in the heating coil means, andreduce transfer of heat at said nucleation site between the heating coilmeans and the liquid in said liquid flow passage, to prevent said liquidfrom reaching said nucleation site and explosively flashing into vaporbetween said nucleation site and the superheated vapor outlet in saidheating coil means.
 4. In a liquid-superheating and -vaporizing deviceas set forth in claim 1, in which said heat dam insert comprises anelongated rod-type insert adapted to be inserted in said heating coilmeans having a restricted liquid flow passage extending between the endsthereof, spaced elongated heat transfer wall members projectingoutwardly at opposite sides of said liquid flow passage, between theopposite ends of said insert and adapted to be disposed in heat transferrelation with the inner surface of said heating coil means, said inserthaving an enlarged central stem member between said heat transfer wallmembers having a smaller diameter than the transverse inner diameter ofsaid heating coil means, to form a heat dam cavity in said insertbetween said central stem member, the inner surface of said heating coilmeans, and said outwardly projecting heat transfer wall members, adaptedto enclose a nucleation site in said heating coil means, a restrictedvapor vent passage in said insert extending across said heat transferwall members in restricted communication between the interior of saidheat dam cavity and said liquid flow passage, whereby said heat damcavity in said insert surrounding said nucleation site is adapted totrap liquid vapor from boiling liquid in said heating coil means at saidnucleation site and restrict the inflow of liquid in said flow passagefrom entering said heat dam cavity and explosively flashing into vaporat said nucleation site.
 5. A liquid-superheating and -vaporizing deviceas set forth in claim 4 in which said spaced elongated heat transferwall members extend helically from end to end of said insert in uniformspaced relation around said enlarged central stem portion with saidrestricted liquid flow passage extending helically around said centralstem member between said helically extending spaced elongated heattransfer wall members, to provide a heat dam cavity helicallysurrounding said central stem member, between said helical wall membersand said enlarged central stem member and the inner surface of saidheating coil means, extending across said nucleation site in the heatingcoil means where liquid in said heating coil means begins to boil andtends to explosively flash into vapor, and a restricted venting passagein said helical dam cavity opening into said helical liquid flow passagebetween said spaced helical wall members, for restricting the inflow ofliquid from said helical liquid flow passage into said helical heat damcavity by the vaporized liquid in said heat dam cavity.